This invention relates to apparatus for controlling the position of conveyors and particularly to improved systems for monitoring and controlling the position of hoists which move articles to be treated along electroplating tanks.
Various mechanisms such as limit switches, bar code readers, and other optical devices have been used for determining the position of conveyors or elevator-conveyors such as hoists along a path to and from work stations. In the electroplating industry, for example, such mechanisms are utilized in controlling the movement of a hoist mounted over or on one side of a line of several tanks so that articles carried by the hoist may be immersed in baths of various tanks according to a desired sequence.
Each of the positioning systems currently in use have disadvantages. For example, mechanical devices such as limit switches which engage code plates in parallel are bulky and not very reliable. Bar code readers and similar optical coding devices depend on the reflectivity of code-generating materials and thus may be subject to errors from accumulation of dust, oil, or mist on the reflective materials or from spurious reflections. Bar code readers also cannot, unless complicated optics are used, identify the position of a hoist when the hoist is stationary. Transmissive optical systems which utilize one or more photoelectric switches traveling with the hoist and a stationary light at each work station are bulky and expensive; they typically require a separate electrical line for each light. Moreover, such systems do not provide positive station identification when the moving hoist is stopped by an on-station light, and positioning errors are possible if an incorrect light is illuminated.
Accordingly, it is an object of the invention to provide an improved system for monitoring the position of a conveyor such as a hoist.
It is a particular object of the invention to provide a system for monitoring and controlling the position of a hoist along a line of electroplating tanks which gives positive identification of station position.
It is an object of the invention to provide a compact, reliable, low cost system for accurately positioning a conveyor such as a hoist along a plurality of work stations.